Anodically polishing zinc surfaces



, Patented July 13, 1954 I 2,683,687 ANODICALLY POLISHING ZINC SURFACES Richard M. Wick, Allentown, Pa., asslgnor to Bethlehem Steel Company, a corporation of Pennsylvania No Drawing. Application April 16, 1949,

Serial No. 88,031-

8 Claims. ((3. 205-1405) This invention relates to the treatment of metal surfaces, and more particularly to a method of anodically treating zinc, or zinc alloy, surfaces.

The principal object of this invention is to brighten zinc surfaces.

Another object is to improve stain resistance of zinc surfaces.

A further object is to improve corrosion resistance of zinc coated metals.

When metals, as iron or steel, are given a coating of zinc, such coating being formed by either the electroplating or immersion process, the zinc surface may be uneven, and quite frequently it has a lacklustre appearance. A dull appearance is particularly prevalent in electroplated zinc, which type of coating has a matte finish. In hot dip zinc coatings, or those made by immersing steel or other metal in molten zinc, the surface of the zinc usually has a mottled, spangled, or otherwise irregular appearance. The irregularity of appearance which usually prevails on untreated zinc surfaces is undesirable in many commercial applications. Numerous means have been used to brighten the surfaces of zinc coatings, in order to give such coatings a more pleasing appearance.

I have found that an unusually bright surface can be obtained on zinc coated articles when such articles are subjected to an anodic electrolyzing treatment, in an electrolyte composed of an aqueous solution of chromic and boric acids. Chromic acid, as used in this invention, may be defined as the anhydride, CrOa. Zinc, zinc alloys in which zinc is the predominant metal, and zinc coated articles all lend themselves admirably to my process. The dull, or unclean, appearance of most hot dip or electroplated zinc surfaces can usually be attributed to high spots. When zinc, or zinc coated articles, acting as the anode, are electrolyzed in an aqueous solution of chromic acid and boric acid under controlled conditions of current density, time and temperature, a small amount of zinc goes into solution. The amount of zinc dissolved depends on the initial roughness of the surface, increasing as the surface roughness increases. The zinc which is dissolved first is that portion of the surface nearest the cathode, and such dissolution removes high spots, thus producing a smoother, more uniform surface, and developing a brighter surface appearance. The addition to the electrolyte of an anode film controlling agent, in the form of boric acid greatly enhances the action of the chromic acid, and makes possible the development of a bright zinc surface.

I have found that excellent results are obtained when a zinc coated article, acting as the anode, is electrolyzed in a water solution containing 250 grams per liter of chromic acid and 12 grams per liter of boric acid. Anodic current density of 150 amperes per square foot is applied to the system for 15 seconds, at an approximate temperature of 80 F. It has been found preferable to use a lead cathode.

A zinc coated article, treated under the conditions described above, will not only have a bright finish, but will show improved stain resistance, as well as improved corrosion resistance under certain corrosive conditions.

While the specific description of my process, as given above, presents certain preferred operating conditions, a wide range of electrolyzing conditions are permissible. For example, good results can be obtained when zinc coated articles are electrolyzed for as short a time as 5 seconds, and the temperature may be held at any point between and 150 F. The process is operable at current densities as low as 25 amperes per square foot, although for current densities below 50 amperes per square foot, the time required for successful treatment must be increased to more than 15 seconds.

Furthermore, current densities as high as 1000 amperes per square foot have been used to produce a good brightening, or polishing, effect on zinc surfaces. When current densities of the order of 1000 amperes per square foot are used, the treating time is extremely short.

Considerable latitude is permissible in the concentrations employed in the electrolyte, good working conditions being obtained with from 200- 300 grams per liter of chromic acid, and from 6- 35 grams per liter of boric acid.

I claim:

1. The process of anodically polishing zinc surfaces which comprises electrolyzing a zinc-coated steel article in an aqueous solution consisting essentially of chromic acid (approximately 250 grams per liter) and boric acid (approximately 12 grams per liter) at a current density of approximately 150 amperes per square foot and at a temperature of from -90 F. for not less than 10 seconds, said zinc-coated steel article acting as the anode.

2. The process of anodically polishing zinc surfaces which comprises electrolyzing a zinc-coated steel article in an aqueous solution consisting essentially of chromic acid (approximately 250 grams per liter) and boric acid (approximately 12 grams per liter) at a current density of approximately 150 amperes per square foot and at a temperature or from '70-90 F. for not less than seconds, electric current passing from said article as anode to a lead cathode.

3. The process of anodically polishing zinc surfaces which comprises electrolyzing a zinccoated steel article in an aqueous solution consisting essentially of chromic acid (200-300 grams per liter) and boric acid (9-15 grams per liter) at a current density of from 500-1000 amperes per square foot for not less than 2 seconds, said zinc-coated steel article acting as the anode.

4. The process of anodically polishing zinc surfaces which comprises electrolyzing a zinccoated steel article in an aqueous solution consisting essentially of chromic acid (200-300 grams per liter) and boric acid (6-35 grams per liter) at a current density in excess of 25 amperes per square foot and not more than 1000 amperes per square foot, said zinc-coated steel article acting as the anode.

5. The process of anodically polishing zinc surfaces which comprises electrolyzing a zinccoated steel article in an aqueous solution consisting of chromic acid (200-300 grams per liter) and boric acid (6-35 grams per liter) at a current density in excess of 25 amperes per square foot and not more than 1000 amperes per square foot, said zinc-coated steel article acting as the anode. v

6. The process of anodically polishing zinc surfaces which comprises electrolyzing a zinccoated steel article in an aqueous solution con- 4. sisting of chromic acid (200-300 grams per liter) and boric acid (6-35 grains per liter) at a current density or from 500-1000 amperes per square foot for not less than 2 seconds, said zinc-coated steel article acting as the anode.

7. The 'process of anodically polishing zinc surfaces which comprises electrolyzing a zinccoated steel article in an aqueous solution consisting of chromic acid (approximately 250 grams per liter) and boric acid (approximately 12 grams per liter) at a current density of approximately amperes per square foot for not less than 10 seconds, said zinc-coated steel article acting as the anode.

8. The process of anodically polishing zinc surfaces which comprises electrolyzing a zinccoated steel article in an aqueous solution consisting of chromic acid (approximately 250 grams per liter) and boric acid (approximately 12 grams per liter) at a current density of approximately 150 amperes per square foot for not less than 10 seconds, electric current passing from said article as anode to a lead cathode.

References Cited in the flle or this patent UNITED STATES PATENTS Name Date Faust Apr. 10, 1945 OTHER REFERENCES Number 

1. THE PROCESS OF ANODICALLY POLISHING ZINC SURFACES WHICH COMPRISES ELECTROLYZING A ZINC-COATED STEEL ARTICLE IN AN AQUEOUS SOLUTION CONSITING ESSENTIALLY OF CHROMIC ACID (APPROXIMATELY 250 GRAMS PER LITER) AND BORIC ACID (APPROXIMATELY 12 GRAMS PER LITER) AT A CURRENT DENSITY OF APPROXIMATELY 150 AMPERES PER SQUARE FOOT AND AT A TEMPERATURE OF FROM 70*-90* F. FOR NOT LESS THAN 10 SECONDS, SAID ZINC-COATED STEEL ARTILCE ACTING AS THE ANODE. 